1. Field of the Invention
The invention relates to a device for calibrating a measuring instrument or serving as a reference target.
2. Background of the Related Art
There are a variety of measuring instruments that utilize electromagnetic radiation to detect physical characteristics or conditions of a material. Such instruments may be used to detect colors or spectroscopic characteristics of objects, or to determine a characteristic of an object. Such an instrument may also be used by medical personnel to diagnose a condition of a patient.
Typically, the instrument will emit radiation at one or more wavelengths, and the emitted radiation is directed toward a target object, or target tissue on a patient. Reflected, transmitted, scattered or emitted radiation that interacts with the target object or tissue, or possibly fluorescent radiation generated by the target object or tissue in response to the emitted radiation, is then detected by the instrument and analyzed to determine characteristics of the target object or tissue, or to determine a condition of the patient.
An example of such an instrument is shown in FIG. 1. The instrument 100 includes a trigger 102 for activating the device. When activated, the device emits radiation at one or more wavelengths from a nose portion 104. Radiation that is reflected, transmitted, scattered or emitted from the target object or patient is then collected by the nose portion 104 and analyzed by a detector of the instrument to determine a characteristic of the target object, or a condition of the patient. The instrument could also be configured such that it receives and analyzes radiation that is naturally emitted from the tissue or object.
Instruments like the one shown in FIG. 1 may require periodic calibration to maintain their accuracy. Over time, various characteristics of the instrument, including the amplitude of the output radiation and the spectral distribution of the output radiation, as well as the total radiation power output, can vary due to environmental conditions, or simple aging of the radiation emitting elements and other elements in the instrument. In addition, a detector of the instrument can have differing sensitivities depending upon environmental conditions or the age of the detector. For instance, a change in temperature of only a few degrees can significantly affect the sensitivity of a radiation detector. For these reasons, the measuring instrument may require a calibration or reference reading prior to taking an actual measurement.
In a calibration operation, the instrument is aimed at a calibration target having known optical properties and radiation is directed toward the target. Radiation is then reflected, transmitted, scattered or emitted by the calibration target and detected by a detector of the instrument. Because the calibration target has known optical properties, the amount and distribution of radiation reflected, transmitted, scattered or emitted by the calibration target and received back in the instrument provides an indication of the radiation directed toward the target. A calibration operation allows a baseline measurement to be taken in order to determine the qualitative end quantitative performance of the light source, detector and other instrument components in order to ensure that the instrument will deliver accurate results.
In a reference operation, the instrument is aimed at a target having known optical properties and radiation is directed toward the target. A predetermined amount of radiation generated by the instrument is reflected, transmitted or scattered by the target, or the radiation causes the target to emit radiation. This reflected, transmitted, scattered or emitted radiation is detected by the detector of the instrument. The results of detection operation can then be used as a standard or reference against which a target object or patient measurement is judged. A reference operation is typically conducted at the point of use. A target object or patient reading could then be derived by determining a difference or ratio between a target object or patient detection operation, and a detection operation conducted on a reference target. Because the optical properties of the reference target are known, variations in light output or detector sensitivity can be accounted for by use of the reference target. This ensures that the instrument continues to deliver accurate results.